Conventionally, sealing of an aluminum-like metal container used in a lithium battery for a mobile device has been performed by pulse seam welding in which a YAG pulsed laser light is irradiated along a junction line between an opening of the container and a sealing member fitted into the opening. More specifically, when sealing an aluminum-like metal container, a YAG pulsed laser light is irradiated to each processing point in a three-level waveform. That is, first, cutting is performed with a high-speed, high-peak pulsed laser, and when the aluminum-like metal material starts to melt, laser power is immediately suppressed to around half to perform final welding. Subsequently, laser power is further reduced to around half to perform annealing in order to relieve residual stress. Welding is performed in this manner using the three-level waveform YAG pulsed laser light because the aluminum-like metal material has a high reflectance and a high thermal conductivity yet a low melting point, and is further characterized by a rapid increase in laser absorptance once melted.
However, while usable for sealing a lithium battery for a mobile device, the three-level waveform YAG pulsed laser light cannot be used for sealing a lithium battery for a hybrid vehicle. This is because while sufficient joint strength can be achieved with a penetration amount of around 0.2 mm in the case of a lithium battery for a mobile device, a penetration amount of around 0.5 mm is required in the case of a large-size lithium battery for a hybrid vehicle, and increasing laser power so as to satisfy such a deep penetration creates spatters.
Meanwhile, conventionally, the use of a superimposed laser light in which a pulsed laser light is superimposed with a CW laser light in pulse seam welding has been proposed (for example, refer to Japanese Patent Laid-Open No. 2004-337881) According to the pulse seam welding using a superimposed laser light, since a pulsed laser light can be irradiated to a processing point that is in a state in which laser light is easily penetratable due to preheating by a CW component, spatterless welding can be performed. A conventional laser apparatus that generates the aforementioned superimposed laser light will now be described.
FIG. 6 is a schematic diagram showing a general configuration of a conventional laser apparatus that generates a superimposed laser light in which a pulsed laser light is superimposed with a CW laser light. The laser apparatus includes a YAG pulsed laser oscillator 101 that oscillates a pulsed laser light having an oscillation wavelength of 1064 nm. A pulsed laser light oscillated by the oscillator 101 passes through an SI optical fiber 102 and is incident to a collimator lens 103. The pulsed laser light collimated by the collimator lens 103 is incident to a dichroic mirror 104.
In addition, the laser apparatus includes a high-output semiconductor laser 105 that oscillates a CW laser light. The CW laser light oscillated by the high-output semiconductor laser 105 is incident to the dichroic mirror 104.
The dichroic mirror 104 superimposes the pulsed laser light from the collimator lens 103 with the CW laser light from the high-output semiconductor laser 105 to generate a superimposed laser light, and causes the superimposed laser light to be incident to a focusing lens 106. The focusing lens 106 focuses the pulsed laser light and the CW laser light superimposed by the dichroic mirror 104 at a processing point.
As shown in FIG. 6, a focus spot 107 of the pulsed laser light having passed through the optical fiber 102 takes a circular shape similar to a core shape of the optical fiber 102. On the other hand, a focus spot 108 of the CW laser light oscillated by the high-output semiconductor laser 105 generally does not take a circular shape, and takes a linear shape as shown in FIG. 6.
When performing pulse seam welding using a superimposed laser light in which the circular-shaped pulsed laser light is superimposed with the linear-shaped pulsed laser light, the major axis direction of the focus spot 108 of the CW laser light is set in a direction aligned with a junction line and the superimposed laser light is relatively moved along a longitudinal direction of the junction line. Accordingly, since the pulsed laser light can be irradiated on a processing point that is in a state in which laser light is easily penetratable due to preheating by a CW component, spatterless welding can be performed.
However, while the superimposed laser light in which the circular-shaped pulsed laser light is superimposed with the linear-shaped CW laser light achieves spatterless welding that satisfies a desired penetration amount when sealing an NiH battery for a hybrid vehicle which uses a steel-like metal container, the superimposed laser light is unable to satisfy a desired penetration amount when sealing a lithium battery for a hybrid car which uses an aluminum-like metal container because heat escapes to the surroundings from a tip of a linear-shaped CW component. In addition, increasing the CW component in order to attain a preheating effect causes deformation of the container and subsequently widens a gap between a wall surface of an opening of the container and the sealing member, which in turn creates a disadvantage that laser light leaks to an inner electrode and damages the inner electrode.
As seen, the conventional laser welding technique is not capable of performing pulse seam welding of thick aluminum-like metal material such as the sealing of an aluminum-like metal container used in a lithium battery for a hybrid vehicle.
Furthermore, the sealing of an aluminum-like metal container used in a lithium battery for a hybrid vehicle has the following disadvantages. Firstly, with aluminum-like metal material, since laser absorption factor varies significantly due to minute differences in surface conditions such as scratches, coarseness and staining, the penetration amount also varies significantly due to surface conditions. On the other hand, during the sealing of an aluminum-like metal container, when laser light penetrates the sealing member, a spatter occurs from the penetrated portion and, in turn, causes a short circuit. Since a short circuit in a lithium battery has a risk of causing a fire, the penetration amount must be controlled so as to prevent the laser light from penetrating the sealing member. Therefore, it is necessary to stabilize the penetration amount even when surface conditions vary.
Moreover, since aluminum-like metal material has a high reflectance and, in particular, has only a laser absorption factor of 7% with respect to YAG pulsed laser light, pulse seam welding of an aluminum-like metal container requires a YAG pulsed laser light in the kW range. Therefore, in order to supply power in excess of 30 kW to an excitation light source to obtain a kW-range YAG pulsed laser light, output current in the order of several hundred amperes must be controlled at the power source for the excitation light source. Meanwhile, since a lithium battery for a hybrid vehicle is about ten times as large as a lithium battery for a mobile device, an increased welding speed is required from a productivity perspective. Increasing welding speed requires reducing a pulse width (welding time) of a pulsed laser light, which in turn requires that a pulse be raised at high speed. Therefore, at the power source for the excitation light source, it is necessary to control the output current in the order of several hundred amperes to be supplied to the excitation light source to a current signal having a high-speed rise and a short pulse width.
However, with a dropper power source, raising a current signal in the order of several hundred amperes at high speed significantly increases equipment size. Therefore, the dropper power source is unsuitable for a laser apparatus to be used to weld aluminum-like metal material. On the other hand, with a chopper/inverter power source that controls output current by switching an internal switch element, clock synchronization is essential. Therefore, a jitter in the order of several ten μs occurs in a chopping clock period signal (drive signal) that drives the switch element and, consequently, a jitter in the order of several ten μs also occurs in the output current. As a result, since reducing the pulse width of a current signal to be supplied to the excitation light source increases the proportion of a jitter component (fluctuation component) and causes a significant power fluctuation in the pulsed laser light, a stable penetration amount cannot be achieved. For example, when the pulse width of the current signal to be supplied to the excitation light source is set to 0.3 ms, a jitter component of 30 μs causes a 10% power fluctuation and penetration amounts also vary by about 10%.
Therefore, with a general dropper power source or a chopper/inverter power source, pulse reduction of a current signal to be supplied to an excitation light source could not be achieved. Consequently, a YAG pulsed laser light whose pulse width is 2 ms or more has been generally used to seal a lithium battery for a mobile device.
In addition, since the pulse width is set to 2 ms or higher, the energy of the YAG pulsed laser light increases. Thus, conventionally, a GI optical fiber could not be used and an SI optical fiber has been used. This is because, with a GI optical fiber, a spatter occurs from an edge surface of a laser exit aperture when the energy of the laser light is increased. However, an SI optical fiber is prone to damages due to on-site adherence of dust and the like during fiber exchange.
Moreover, an SI optical fiber has a small aperture of 0.6 mm to 0.4 mm. Meanwhile, since a lithium battery for a hybrid car has a large size, as far as the fitting relationship between the opening of the container and the sealing member is concerned, the gap between a wall surface of the opening and the sealing member is larger in comparison to a lithium battery for a mobile device. Therefore, when using an SI optical fiber with a small aperture, a wide spot diameter must be set for the pulsed laser light. As a result, power transmissibility to a processing point is reduced in comparison to a wide-aperture GI optical fiber.